In the conventional art, a plurality of communication devices such as ECUs (Electronic Control Units) having a communication function are mounted on a vehicle and then mutual message exchange is performed between the plurality of communication devices so that control processing of diverse kind is realized in the vehicle. In the communication in the vehicle, the CAN (Controller Area Network) protocol is employed widely. A communication system of CAN protocol has a network configuration of bus type in which a plurality of communication devices share a communication line. As for the number of communication devices allowed to be connected to one bus, an upper limit is set forth in the CAN protocol and a further limit is placed depending on the communication amount of the bus and the like. In order that communication devices in a number exceeding these limits should be allowed to be mounted, the bus need be divided by using a gateway device.
In Japanese Patent Application Laid-Open No. 2005-159568, a gateway device is proposed in which in communication performed between mutually different intra-vehicle networks through the gateway device, when the transmission timing is delayed owing to a communication error or the like, data is transmitted reliably in descending order of priority. In the gateway device, IDs (IDentifiers) indicating transmission destinations contained in data to be relayed and transmitted are stored in advance. When plural pieces of data are to be transmitted next, the gateway device determines the order of transmission on the basis of the priorities of the IDs.